Kinetics and mechanism of alkyl transfer from organocobalt(III) to nickel(I): Implications for the synthesis of acetyl coenzyme A by CO dehydrogenase

Reaction of CH3Co(dmgBF(2))(2)L (dmgBF(2) = (difluoroboryl)dimethylglyoximato); L = py, PEt(3)) with 2 equiv of [Ni(tmc)]OTf (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane; OTf(-) = CF3SO3-) gave Co(dmgBF(2))(2)pyy(-), [Ni(tmc)]QTf(2), and [Ni(tmc)CH3]OTf in 80% yield. The overall transformation provides the first model for the transfer of a CH3 group from methylcobalamin to the Ni-containing enzyme carbon monoxide dehydrogenase during acetyl coenzyme A synthesis. RRSS-[Ni(tmc)CH3](BAr'(4)) (BAr'(-)(4)= B(3,5-(CF3)(2)C6H3)(4)(-)) has been characterized by X-ray diffraction. The prohucts and 1CH(3)Co(dmgBF(2))(2)L:2[Ni(tmc)]OTf stoichiometry of the reaction are consistent with a three-step mechanism initiated by electron transfer from [Ni(tmc)]OTf to CH3Co(dmgBF(2))(2)L. The second step is rapid CH3-Co- bond homolysis yielding Co(dmgBF(2))(2)L(-) and CH3 .; then the CH3 radical is captured by the second equivalent of [Ni(tmc)]OTf, yielding [Ni(tmc)CH3]OTf. Radical clock experiments have corroborated the production of free radicals. Reaction of (5-hexenyl)Co(dmgBF(2))(2)L with [Ni(tmc)]OTf, followed by hydrolysis of the organonickel products, gave methylcyclopentane, consistent with the formation and cyclization of the 1-hexenyl radical. The second-order rate constants measured by stopped-flow experiments parallel the relative radical stabilities: R = CH3 (2.43 x 10(3) M(-1) s(-1)) < C2H5 (7.88 x 10(3) M(-1) s(-1)) < CH(CH3)(2) (19.2 x 10(3) M(-1) s(-1)), L = py (2.43 x 10(3) M(-1) s(-1)) < PEt(3) (5.01 x 10(3) M(-1) s(-1)). During the course of these studies the following molecules were also characterized by X-ray diffraction, CH3Co(dmgBF(2))(2)L, L = PEt(3), py, H2O.